1. Field of the Invention
The present invention relates to an apparatus for transferring liquefied gas.
2. Description of the Prior Art
Generally, glandless pumps such as pumps having a canned type motor or wet type motor are used in apparatus for transferring liquefied gas from a closed tank to a container or the like, because the glandless pumps are easy to assemble and completely free from the problem of leakage as compared with pumps having mechanical shaft seals. The glandless pumps are also advantageous as compared with pumps having oil immersion type motors, because insulation oil and a device for supplying insulation oil are not necessary and because the handling is easy. For these reasons, the glandless pumps are now finding increasing use.
The liquefied gas in the glandless pump is heated by the heat generated as a result of pump loss and motor loss. This, however, does not impose any problem because the liquefied gas does not stagnate in the pump but steadily flows through the pump during the operation of the pump, so that the amount of heat received per unit of volume of the liquefied gas is so small that the liquefied gas in the pump is never heated excessively.
However, in the case where the transfer of the liquefied gas is made discontinuously and frequently by intermittent and frequent start and stop of the pump as in the case of an LP gas station, the liquefied gas in the pump is evaporated to hinder the smooth transfer of the liquefied gas.
Namely, as the pump is stopped after completion of the transfer of liquefied gas, the liquefied gas stagnated in the pump is heated by the residual heat, so that the amount of heat received by a unit volume of the liquefied gas is drastically increased. In consequence, the liquefied gas is heated excessively and evaporated rapidly. The vapor then escapes, as time elapses, into the closed tank through the pump suction pipe and into the upper part of the pump discharge pipe, and then the pump itself is filled with the liquefied gas. However, if the pump is started without removing the vapor from the pump, a phenomenon called cavitation takes place to make the pump idle with a high level of noise and fail to transfer the liquefied gas. In consequence, the pump cannot fully exert its performance, and the bearing is worn down rapidly due to insufficient lubrication, resulting in trouble.
In the case of an immersion type glandless pump, e.g. a liquid immersion type canned motor pump which is contained in a closed tank 2 as shown in FIGS. 1 and 2, the following problems are encountered in addition to the problems mentioned above. Namely, if the liquid level in the closed tank 2 is lowered below the level of the bearing 10a or bearing 10b or the bearing 11, the bearing 10a or 10b or 11 cannot be immersed in the liquefied gas 5 during suspension of operation of the pump. Therefore, the bearing 10a, 10b or 11 has to work without being lubricated over a period of several seconds from the start up of the pump 4 till the recirculation of the liquefied gas 5 to the bearing 10a, 10b or 11, so that the wearing down of the bearing takes place abnormally rapidly. To ensure the safe operation of the pump, therefore, it is necessary to make a periodical disassembling for inspection at an early stage.
The disassembling of the liquid-immersed type canned motor pump 4 is very troublesome, time-consuming and expensive because the pump 4 cannot be demounted without completely removing the liquefied gas 5 in the closed tank 2 and because the opening examination of the tank 2 is necessary once it is opened to the atmosphere.
To avoid these problems, as shown in FIGS. 1 and 2 showing a canned motor pump 4 as an example of the immersion type glandless pump and in FIG. 4 showing a canned motor pump 4 as an example of the ground mounting type glandless pump, the pump 4 is operated continuously even when the transfer of the liquefied gas 5 to a container or the like is not necessary, although the valve 7 is kept closed. By so doing, the liquefied gas 5 is sucked into the pump 4 from the closed tank 2 and is returned to the closed tank 2 through the discharge pipe 1 and a by-pass passage 3, to maintain a continuous flow of the liquefied gas through the pump, thereby to prevent the evaporation of the liquefied gas 5 in the pump 4. This method, however, is not preferred because the running cost is very high due to the continuous running of the pump even in the period in which the transfer of liquefied gas is not necessary.